1. Field of the Invention
This invention is related in general to scanning tunneling and scanning force microscopy. In particular, it describes a method and a mechanism for always operating the scanning means of such microscopes in coaxial alignment with the instrument's probe irrespective of the portion of sample being scanned.
2. Description of the Related Art
During the last decade scanning tunneling microscopes (STM's) and scanning force microscope (SFM's; also known as atomic force microscopes) have evolved into powerful tools in science and technology for measuring microscopic features and physical properties of materials. Scanning tunneling microscopy is based on the principle of quantum mechanical tunneling of electrons between two electrodes, such as an atomically sharp metal tip and a sample, under an applied electric field. A potential is applied through a feedback control system that maintains a constant current between the tip and the sample by controlling the vertical distance between the two. In one configuration, the tip is held stationary while the sample is mounted on piezoelectric ceramic material that is capable of moving the sample in the x, y and z directions with respect to the tip by the application of electric fields to the ceramic. In another configuration, the sample is stationary and the tip is mounted on a scanning piezoelectric ceramic. In either case, the accurate positioning of the tip in the x, y and z directions relative to a target point on the sample allows high-resolution point measurements of surface topography, electrical conductivity, electronic and atomic structure, and chemical composition.
Scanning force microscopy functions on the principle of a conventional stylus profilometer having a sharp point mounted on a flexible cantilever and moved across the surface of a sample. The motion of the stylus is correlated to a property being observed as a target point in the sample is being scanned by means equivalent to the ones used by STM's. Scanning force microscopy is used to study various interactions between a probe and a surface, such as interatomic, frictional, magnetic, electrostatic, and adhesion forces In addition, SFM's are used to produce high-resolution images for both conductive and insulating materials. As in the case of STM's, either the probe or the sample is mounted on a scanning mechanism that allows the relative motion of the two along the surface of the sample. Scanning is conducted under one of three modes of operation. In the first mode, the deflection of the cantilever is held constant by adjusting the vertical position of the probe or the sample with a feedback control loop (constant force mode). In the second mode, the sample is kept at a constant height and the variation of the deflection of the cantilever during scanning is used to produce a topography of the surface. In the third mode, the cantilever is modulated near its resonance frequency, such as by a piezoelectric unit, and the amplitude or phase change of the vibration is monitored to produce a measurement of the distance between the probe and the surface of the sample.
Scanning tunneling and atomic force microscopes based on these principles are well known in the art and are described in detail in the literature. See, for example, Jahanmir, J. et al., "Scanning Microscopy, " Vol. 6 No. 3, 1992 (pp. 625-660). The present invention is directed to the class of scanning tunneling and atomic force microscopes that utilize a fixed probe interacting with a sample mounted on a piezoelectric mechanism that provides the scanning action required for the operation of the system. A typical such system for an STM is shown schematically in FIG. 1 for illustration. A fixed probe 10 mounted on a rigid support block 12 is kept within tunneling distance of the surface 14 of a sample 16 mounted on a piezoelectric element or tube 18 by a conventional feedback mechanism. The tunnel current I resulting from a tunnel voltage P applied between the probe and the sample is converted to an output voltage V by a current detection circuit 20. The output voltage V is compared to a setpoint reference value S to produce an error signal E, which in turn is converted to a control voltage by control circuitry 22 that adjusts the z position of the ceramic to minimize the error. The control voltage is stored as a function of x and y positions and related to the topography of the surface 14. Motion of the sample in the x and y directions is provided, usually in raster fashion, by scanning voltages applied to the piezoelectric element 18 (not illustrated in the FIGURE).
Thus, the piezoelectric element 18 provides the vertical motion as well as the scanning motion of the sample 16 with respect to the fixed probe 10. Typically, piezoelectric ceramics are either mounted in an orthogonal tripod arrangement for independently scanning the x, y and z directions or consist of a single-tube ceramic sectioned into four equal parts parallel to the axis of the tube. Different voltage potentials applied to the various sections cause different degrees of expansion of the ceramic sections that result in x-, y- and z-directional movement of a sample stage connected to the top of piezoelectric element. The movement so produced in the x-y plane provides the scanning of the surface of the sample 14 by the tip of the stationary probe 10 (see FIG. 1).
One of the design specifications for a typical STM system is that the tip-to-sample position control be better than the resolution desired for the application of interest. Thus, for example, for atomic imaging the tip position has to be resolved 0.1 angstrom vertically and 1 angstrom laterally and the dynamic range required is a few thousand angstroms in the x, y and z directions. Piezoelectric ceramics, which are capable of position control within 0.1.ANG. and have a dynamic range of several micrometers, satisfy the scanning requirements of most systems once the target of interest in a sample is positioned directly under the point of the probe. Between scanning operations, coarse positioning of the tip with respect to the sample is provided by translational mechanisms that move either the probe or the piezoelectric/sample assembly and permit the precise positioning of the tip of the probe on the target area on the surface of the sample, typically with the aid of optical instrumentation. The target area is then scanned by the piezoelectric action described above.
During horizontal scanning, the piezoelectric tube 18 of typical STM or SFM apparatus provides lateral movement of the sample stage connected to it by bending in the direction of motion as a result of the net effect of the voltages applied to the ceramic's various sections. Therefore, such bending introduces a tilt .alpha. in the position of the sample 16 that becomes progressively pronounced as the limits of the scanning range of the piezoelectric element 18 are approached, as illustrated in FIG. 2 (in exaggerated fashion for clarity).
This tilt, which is typically in the order of seconds of a degree, is the source of several problems that the present invention is directed at solving. The first problem is a material reduction of the vertical operating range between the probe 10 and the sample 16 when the probe is aligned with a peripheral portion of the sample, as illustrated in FIG. 3. Since the probe 10 is positioned over the target area by the translational mechanism 24 (FIG. 2) but is stationary during scanning, its distance d from the sample is obviously affected by any tilt .alpha. in the plane of the sample and the variation is progressively increased as the probe is further removed from the vertical axis A of the piezoelectric element (in its relaxed state). Thus, for example, when the sample is raised toward the probe by the tilting action resulting from scanning toward the left of the sample (requiring-the piezoelectric tube to bend toward the right), the probe may press against the sample and produce highly distorted images (FIG. 3). When the sample is lowered away from the probe by the tilting action, as illustrated in phantom line in FIG. 3, the probe may be outside the vertical range of the piezoelectric element and again produce highly distorted images or no images at all.
The second problem associated with the prior art is related to the hysteresis that all piezoelectric elements show during the electromechanical cycles that produce scanning. In order to position the probe at the appropriate distance from the sample, because of the tilt .alpha. in the plane of the sample, the vertical motion required of the piezoelectric element is greater when the probe is not aligned with the vertical axis A of the scanning piezoelectric element. Accordingly, all hysteresis effects experienced between scanning operations become more pronounced as the target point is moved away from the axis A of the piezoelectric element and are more difficult to correct by means of standard electronic circuitry.
A third problem is related to the nonlinear response of piezoelectric elements to applied voltages. As in the case of hysteresis, the tilt of the sample also causes the piezoelectric element to operated in less linear regions when the scanning is performed over a target located away from the vertical axis A of the piezoelectric ceramic. This higher degree of nonlinearity distorts the voltage readings and introduces an additional source of error that must be corrected by numerical or other means. This invention provides simple solutions that materially improve the effects of these problems in the operation of STM's and SFM's.